Windows 7: All updates fail with 0x80071a91

Hello,

I have since about 2 months a very annoying problem. I cannot update
my Windows 7 Professional (64bit) anymore. Any attempt as standalone
install or through Windows Update gives osmething like this in the
WindowsUpdate.log:
2010-06-20 20:26:47:783 5192 854 Misc ===========
Logging initialized (build: 7.3.7600.16385, tz: +0200) ===========
2010-06-20 20:26:47:784 5192 854 Misc = Process:
C:\Windows\system32\wuauclt.exe
2010-06-20 20:26:47:784 5192 854 Misc = Module:
C:\Windows\system32\wuaueng.dll
2010-06-20 20:26:47:783 5192 854 Handler :::::::::::::
2010-06-20 20:26:47:784 5192 854 Handler :: START ::
Handler: CBS Install
2010-06-20 20:26:47:784 5192 854 Handler :::::::::
2010-06-20 20:26:47:785 5192 854 Handler Starting
install of CBS update B0B6CF4D-30CF-4C12-8666-D5EAFA83E8A6
2010-06-20 20:26:47:786 5192 854 Handler CBS package
identity: Package_for_KB981332~31bf3856ad364e35~amd64~~6.1.1 .0
2010-06-20 20:26:47:787 5192 854 Handler Installing
self-contained with
source=C:\Windows\SoftwareDistribution\Download\16 161488fce80bf7a5bf7a6122602a88\windows6.1-kb981332-x64.cab,
workingdir=C:\Windows\SoftwareDistribution\Downloa d\16161488fce80bf7a5bf7a6122602a88\inst
2010-06-20 20:26:52:692 5192 10b8 Handler FATAL: CBS
called Error with 0x80071a91,
2010-06-20 20:26:52:726 5192 854 Handler FATAL:
Completed install of CBS update with type=0, requiresReboot=0,
installerError=1, hr=0x80071a91
2010-06-20 20:26:52:726 5192 854 Handler :::::::::
2010-06-20 20:26:52:726 5192 854 Handler :: END ::
Handler: CBS Install
2010-06-20 20:26:52:726 5192 854 Handler :::::::::::::

I checked already a lot ob KB articles, I also tried
Windows6.1-KB947821-v5-x64.msu, but there seems to be no easy way out.
Does somebody have an idea how this is fixed?

Many thanks and bye,
Stephan